Dimethyl carbonate (DMC) is an important intermediate and is widely used in industry. Owing to its low toxicity, dimethyl carbonate is considered a “green” chemical product with bright development prospects. This increasing focus is mainly due to the bio-degradability, with a low bioaccumulation as well as its low toxicity. Significant amount of work is being carried out to develop environmentally safer route for the synthesis of DMC.
Dialkyl carbonates like dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), dibutyl carbonate (DBC) are having potential applications in lithium battery as an electrolyte and can be prepared from urea and corresponding alkyl alcohol. In all cases first step will be the conversion of urea and alkyl alcohol to corresponding carbamate derivative. This is a non-catalytic equilibrium limited reaction. The conventional processes use stirred reactors which often limit the conversion and adversely influence selectivity.
Alkyl carbamate (for example methyl carbamate MC) synthesis by urea alcoholysis has been reported in literature with both catalytic/non-catalytic routes. The non-catalytic processes known in the art for urea alcoholysis however produced low yields of carbamates since these are equilibrium reactions. The further developments used tubular reactors using catalyst to enhance the conversion rate as well as yield.
In literature several reactor configurations have been reported for this reaction mostly operating in batch, semi batch mode in stirred vessel type of reactors.
U.S. Pat. No. 8,338,634 discloses a method for the synthesis of alkyl carbamates by reacting urea with a hydroxyl group containing compound in the presence of a catalyst. The catalyst comprises of a catalytically active component and a catalyst support. The reaction is carried out in a reactor, which may be a tubular reactor. The reaction may be continuous, semi-continuous or batch-wise. The ammonia formed during the reaction is removed from the reactor by appropriate means to shift the reaction equilibrium to the product side.
US20150315134 discloses a process for synthesis of methyl carbamate and dimethyl carbonate (DMC) in a reactor. The urea and methanol or methyl carbamate and methanol are fed into a reactor. The feed are put into a reactor vessel containing expanded slurry bed of solid catalyst particles suspended in a suspension liquid or in a packed bed of solid catalyst.
Since the synthesis of alkyl carbamates using urea and alcohol is a reversible reaction the equilibrium conversion depends on the alcohol to urea mole ratio. If byproduct ammonia is removed from the reaction mixture the equilibrium conversion is increased further. If sufficient alcohol to urea mole ratio is provided substantial urea conversion can be attained without removing ammonia.
The present inventors observed that there is a scope to provide urea alcoholysis reaction in a tubular reactor with simplified configuration/integrated system, in catalyst free environment, with near complete conversion of urea by stripping the ammonia formed and thus removing the equilibrium limitation.